This invention relates to the use of a charged coupled device (CCD) for the direct detection of scattered x-rays used in the determination of the structure of a protein crystal.
Currently, the scattered x-rays used to determine the structure of a crystalline material are detected using film or in the alternative a scintillating or phosphorescent material which when illuminated by a scattered x-ray gives off a visible light. In the latter case the phosphor is coupled with a device which is capable of detecting the light given off by the phosphor and converting it into electronic data which is used to determine the position and intensity of the scattered x-rays. Often a CCD is used in conjunction with the phosphor to determine the position and frequency of the x-rays which are scattered off of a crystallized version of a molecule. With a CCD a single photon can generate a measurable electrical charge. The CCD employs a two dimensional array of cells or pixels, each of which acts as an independent x-ray detector, thus, allowing many scattered x-rays to be stored in the cells of the CCD and then read and recorded.
Generally, CCD-based imaging devices compromise sensitivity and resolution due to smearing and spatial distortion of the image caused by the presence of the scintillating or phosphorescent material between the crystal and the CCD. This is due to the alteration of the path of the x-rays upon interacting with the scintillating or phosphor material. Since, after the x-ray interacts with the light emitting material, the photons emitted do not necessarily follow the exact path of the initial x-ray. This results in a decrease in the resolution or a smearing of the measured point of impact on the CCD of two successive x-rays traveling along the same or close to the same path.
As related above, the prior art CCD-based imagery employs an intermediate light generating transducer, such as a phosphor, to transform the incident x-rays to a light source. The photons from the light source are converted by the CCD to electrical signals. The conversion step of using the phosphor was instituted with the x-rays since the majority of x-rays striking a typical CCD would pass through the CCD without detection. This results from the CCD being too thin.
An alternate method of detection was proposed by Antonuk et al. as described in U.S. Pat. No. 5,262,649, in which, the use of a CCD is avoided in lieu of a thin film amorphous silicon device. Antonuk's patent teaches a thin-film, flat panel, pixelated detector array which serves as a real-time digital image and dosimeter for x-rays or gamma rays. The detector is a plurality of photodiodes made of hydrogenated amorphous silicon arrayed in columns and rows on a glass substrate. Each photodiode is connected to a thin film field effect transistor also located on the glass substrate; the combination of which forms one pixel. For megavoltage beams, a photon to electron conversion layer is located directly above and in contact with a phosphor or scintillating layer. Since each sensor is adjacent to and connected to its corresponding field effect transistor, the area available for detection in reference to the total area of the detector is drastically reduced.
Applicants in their invention provide for the direct detection of the scattered x-rays without an intermediate phosphor or scintillation layer. By using a large area, thick CCD device, applicants can detect the scattered x-rays directly and on a real-time basis. Use of a thick CCD as a direct detection device for use in detecting breast cancer is described in U.S. patent application entitled "High Resolution Mammography", Ser. No. 08/697,536, filing date Aug. 26, 1996, now U.S. Pat. No. 5,742,659, which is incorporated herein in its entirety by reference.
One object of this invention is provide a device which is capable of directly discerning, in real-time, the frequency and position of x-rays scattered by a protein crystal initially subjected to an incident x-ray beam.
Another object of this invention is the employment of a large area, thick CCD to provide a high resolution image of the pattern generated by the scattered x-rays resulting from the interaction of the protein crystal and the incident x-ray beam.
Additional advantages, objects and novel features of the invention will become apparent to those skilled in the art upon examination of the following and by practice of the invention.